Exhaust gas purification systems which have been developed for purifying NOx in diesel engine exhaust gas include, for example, a urea SCR system using a selective catalytic reduction (SCR) (see, for example, Japanese Patent Application Laid-open No. 2000-27627 and Japanese Patent Application Laid-open No. 2005-83223).
This urea SCR system is configured to supply an aqueous urea solution (with a urea concentration of 32.5%), for example, serving as a liquid reducing agent to an upstream part in the direction of exhaust gas flow of the SCR device, so that urea is hydrolyzed to generate ammonia by heat of the exhaust gas, and NOx is reduced by this ammonia on a SCR catalyst.
As shown in FIG. 4, an exhaust pipe 3 is connected to an exhaust manifold 2 of a diesel engine 1. A SCR device 4 for NOx reduction is connected to the middle of the exhaust pipe 3, and a reducing agent supply pipe section 8 having an injection nozzle 7 for supplying a liquid reducing agent (urea aqueous solution) 5 to a SCR catalyst 6 in the SCR device 4 is connected to an upstream part in the direction of exhaust gas flow in the SCR device 4.
A liquid reducing agent storage tank 10 is connected to the injection nozzle 7 of the reducing agent supply pipe section 8 via a liquid reducing agent supply pipe 9. This liquid reducing agent storage tank 10 is configured to pressure feed the liquid reducing agent 5 from a liquid feeding pump 11 to the injection nozzle 7 via the liquid reducing agent supply pipe 9.
The liquid reducing agent storage tank 10 is provided with a filler pipe 12, so that the tank 10 is refilled with the liquid reducing agent 5 through the filler pipe 12 after removing a cap 13 provided on the filler pipe 12.
This liquid reducing agent storage tank 10 is installed in a small place on a side of a vehicle in the vicinity of the exhaust pipe 3, and the filler pipe 12 is attached to an upper edge 10e of the liquid reducing agent storage tank 10 on the vehicle side such that the filler pipe 12 is inclined obliquely upward to facilitate the injection of the liquid reducing agent 5.
Further, a breather pipe 14 is provided on top of the liquid reducing agent storage tank 10 in order to introduce air from the outside and to discharge air according to change of the liquid surface level in the tank.